Hitoshi Nagayama

The effect of grain boundary segregation on superplastic behavior in cation-doped 3Y-TZP

Abstract Superplastic behavior in 3Y-TZP is sensitively affected by 0.2 mol% of cation-doping. The flow stress in unsoluble cation-doped 3Y-TZP is correlated well with ionic radius of dopant-cation as well as that in soluble cation-doped 3Y-TZP. Results in HREM-EDS analysis indicates that the dopant effect on the superplasticity must be caused by the grain boundary segregation of dopant cation.

Superplastic Behavior in Small Amount of Ge-Ti Co-Doped TZP

Superplatic behavior in GeO2 or TiO2 doped and Ge-Ti co-doped 3mol% yttria doped tetragonal zirconia polycrystals (TZP) are examined under a uniaxial tension in air at 1400 and an initial strain rate of 1.3 10 -4 s -1 . Diffsivity on static grain growth behavior is enhanced by Ge-Ti co-doping. Dependence of the flow stress on the amount of co-doped Ge-Ti is close to that in Ge doped TZP, but much lower than that in Ti-doped TZP. The elongation to failure is highly improved by the co-doping in comparison with Ge or Ti doping. The largest nominal strain of 988% was obtained in 2mol%GeO2-2mol%TiO2 co-doped TZP.

Small Dopant Effect on the Superplastic Flow and Failure in 3Y-TZP

Superplastic behavior in 0.2 mol% of cation doped fine-grained yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) was examined with a special interest in cation-segregation along grain boundaries. The flow stress of cation doped 3Y-TZP is correlated well with the ionic radius of doped cation independently of the solubility of each cation in TZP. HREM-nano probe EDS analysis indicates that the dopant effect on the superplasticity is caused mainly from the grain boundary segregation of doped cation.

Criterion for High Temperature Failure and Grain Boundary Chemistry in Superplastic TZP

Temperature and strain rate dependence on high temperature elongation to failure in fine-grained ceramics is phenomenologically explained from grain growth behavior during deformation and the superplastic flow behavior. The elongation to failure at temperatures between 1573 and 1773 K was analyzed for 2 mol%TiO2 and 2 mol%GeO2 co-doped tetragonal zirconia polycrystal (TZP), which exhibits excellent high temperature ductility. The improvement in the high temperature ductility in TZP is attributed to dopant cation segregation in the vicinity of the grain boundaries. The phenomenological analysis revealed that co-doping of Ti and Ge cations increases the grain size at the time of failure, as a parameter to describe a limit of an accommodation process for superplastic flow. The parameter of the critical grain size at the time of failure correlates well with the value of overlap population in cation-doped TZP model cluster obtained from a first-principle molecular orbital calculation. The covalent bond at the grain boundaries plays a critical role in the high temperature tensile ductility of TZP.

GeO2-Doping Dependence of High Temperature Superplastic Behavior in 3Y-TZP

Superplastic behavior in a fine-grained, GeO2-doped 3 mol% yttria-stabilized tetragonal zirconia polycrystal (3Y-TZP) with the dopant level of 0.2 to 3 mol% was examined at 1400°C under an initial strain rate of 1.3×10-4 s-1. Microstructure and chemical composition at the grain boundaries were examined by high-resolution transmission electron microscopy (HRTEM) combined with an X-ray energy dispersive spectrometer (EDS). No secondary phase was observed along the grain boundaries, though EDS analysis indicated the segregation of Ge cations along the grain boundaries. The Ge content at the grain boundaries tends to increase with increasing the total amount of GeO2 addition, but saturate over the doping level of 2 mol%. Dependence of flow stress reduction on the total amount of GeO2 addition has a good correlation with Ge content at the grain boundaries. This fact indicates that the GeO2-doping effect on the flow stress in 3Y-TZP is caused mainly from the grain boundary segregation of Ge cations.